1. Field of the Invention
The present invention relates to a measuring apparatus and a measuring method for measuring an inductance included in a semiconductor device. More specifically, the present invention relates to a measuring apparatus and a measuring method capable of measuring a parasitic resistance and a parasitic capacitance of an inductor in addition to the inductance of this inductor.
2. Description of the Background Art
A voltage control oscillator (VCO) or a PLL (Phase Locked Loop) is generally used in the RF (Radio Frequency) analog device or in the high-speed digital device. The RF analog device uses the VCO and the PLL for generating communication carriers. Furthermore, the high-speed digital device uses the VCO and the PLL for synchronizing the data.
The VCO generates the waves based on an LC oscillation between an inductor L and a variable capacitor C. The resonance frequency is inversely proportional to {square root over (LC)}. Using the variable capacitor C makes it possible to set the oscillation frequency to a predetermined value.
Furthermore, to lock the oscillating condition at a predetermined frequency, the PLL is used. The PLL is a circuit for detecting a variation in an input frequency and correcting it. The PLL adjusts the oscillation frequency of VCO in accordance with a correction signal.
One of inductor elements disposed on the semiconductor devices is a spiral inductor. The spiral inductor is formed on a semiconductor substrate so that a metallic wiring is disposed in a spiral pattern when seen from above.
The Q value is generally known as an index representing an efficiency of an inductor. The Q value is defined as a ratio of an energy loss per one oscillation period with respect to an energy stored in an inductor (or a capacitor). The larger the large Q value is, the more the efficiency increases.
In the oscillator, the jitter decreases with increasing Q value when plotted on a graph with an ordinate representing the amplitude of an oscillating wave and an abscissa representing the time axis. The jitter represents a time deviation. The larger the Q value is, the smaller the time deviation of an oscillating wave becomes.
Furthermore, when the frequency spectrum of an oscillating wave is shown with respect to an abscissa representing the frequency, the half value width of a spectrum becomes small with increasing the Q value. In other words, the larger the Q value is, the faster the convergence to a predetermined frequency occurs. Therefore, the Q value is a factor determining the oscillation performance of VCO.
Hereinafter, the explanation for the above-described spiral inductor resumes. According to the above-described definition, the Q value becomes small with increasing energy loss per one oscillation period. Three factors for causing the energy loss of a spiral inductor are conductor loss, electrostatic induction loss, and electromagnetic induction loss (i.e., eddy current).
The conductor loss represents an energy loss caused by a resistance of a metallic wiring constituting the spiral inductor. The electrostatic induction loss represents an energy loss of current consumed in a semiconductor substrate when the current flows in the semiconductor substrate via a parasitic capacitance developed between the metallic wiring constituting the spiral inductor and the semiconductor substrate. The electromagnetic induction loss (i.e., eddy current) represents an energy loss of eddy current consumed in the semiconductor substrate when the eddy current appears in the semiconductor substrate due to the electromagnetic induction caused in response to a time variation of current flowing in the metallic wiring during an operation of the spiral inductor.
As described above, the mechanism of energy loss caused in the spiral inductor is complicated. Although obtaining its inductance is not easy, a variety of measuring methods have been conventionally proposed.
For example, Japanese Patent Application Laid-Open No. 2000-28662 (col. 7 to col. 9, and FIG. 1) discloses a technique for obtaining a reflection coefficient by measuring a scattering parameter (i.e., S parameter) defined on a matrix combining an incident wave and its reflected wave with respect to the spiral inductor, and then obtaining an inductance and a resistance value of the spiral inductor by plotting it on a Smith chart.
Furthermore, Japanese Patent Application Laid-Open No. 2-300670 (1990) (col. 4 to col. 6, and FIG. 1) discloses a technique for measuring a voltage by supplying a variable current to an inductor and calculating an inductance based on a measured voltage value.
The S parameter is measured in a high-frequency region exceeding 1 GHz. Hence, the measurement of S parameter tends to be adversely influenced by the noises residing in the measuring environment. For example, a pressure caused when a needle (or probe) of a measuring apparatus is brought into contact with a connecting pad of a semiconductor device, or a tiny variation of a voltage value given from the measuring apparatus to the device, gives a great influence to a measured value. Under the condition that the needle is placed on the connecting pad, the pressure applied on this needle may be different according to each measurer. This possibly causes a problem that the reflection of a signal changes and hence a measuring error will be caused. Another problem is that several hours or a comparable waiting time will be required until the measuring apparatus is stabilized.
Furthermore, performing the measurement in the high-frequency region makes a parasitic element formed due to the electrostatic induction or electromagnetic induction become a disturbing factor in increasing the measuring accuracy of inductance.
In view of the above-described problems of the prior art, the present invention has an object to propose an inductance measuring method which is simple and accurate.
To accomplish the above and other related objects, the present invention provides an inductance measuring method for measuring an inductance of an inductor disposed on a semiconductor substrate, including the following first to third steps. The first step is to supply a current pulse to the inductor by applying a periodic voltage to a control electrode of a control transistor. The control transistor has a main electrode connected to one end of the inductor. The second step is to measure a current during a rising term and a falling term of the current pulse by using a first measuring system connected to the other end of the inductor. And, the third step is to measure a current during the rising term and the falling term of the current pulse by using a second measuring system connected via a resistor to the main electrode of the control transistor. The first measuring system includes a first measuring line for measuring a current during the rising term of the current pulse, and a second measuring line for measuring a current during the falling term of the current pulse. The second measuring system includes a third measuring line for measuring a current during the rising term of the current pulse, and a fourth measuring line for measuring a current during the falling term of the current pulse. With the above features, an inductance of the inductor is measured by separating the measurement of the current flowing across the inductor into a measurement of the current during the rising term of said current pulse and a measurement of the current during the falling term of the current pulse.
Accordingly, the current pulse having a predetermined period flows via the control transistor to the inductor. The current flowing across the inductor is measured by using the first and second measuring systems. This makes it possible to separate the measurement of current into two stages; i.e., a measurement of current flowing during the rising term of the current pulse and a measurement of current flowing during the falling term of the current pulse. When the current pulse flowing across the inductor has periodicity, it is possible to prevent an accumulation or integrated value of voltage during the rising term of the current pulse, where a voltage generated between both ends of the inductance is positive, from being canceled by an accumulation or integrated value of voltage during the falling term of the current pulse, where the voltage generated between both ends of the inductance is negative. Thus, it becomes possible to accurately obtain the inductance.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.